The present invention relates to a molybdenum silicide material with high strength even at lower temperatures, such as at room temperature, and high strength at high temperatures, as well as good oxidation properties.
Metallic materials have very good physical properties at lower temperatures, but have worse physical properties at higher temperatures. This is valid even for alloyed metallic material. In practice metallic materials cannot be used at temperatures higher than approximately 1100xc2x0 C.
Ceramic materials such as SiC have good high temperature properties, but are for brittle in the whole range of temperatures.
Intermetallic materials, such as Ni-, Ti- and Fe-aluminides have good high temperature properties, but their practical application temperature is below 1000xc2x0 C.
Silicides, such as molybdenum silicides, are attractive intermetallic materials with very good physical properties at high temperature. They have also very good oxidation and corrosion properties due to that a layer of glass is formed on the surface, which forms a protective film.
However, like most intermetallic materials, these have low ductility and low rupture strength at room temperature. The brittle-ductile transformation temperature (BDTT) for suicides is normally high, for instance above 1000xc2x0 C. for MoSi2. Below the transformation temperature suicides are extremely brittle and have low impact strength. Above the transformation temperature silicides outperform metals with regard to elongation properties and tension reducing properties. Since suicides undergo creep and plastic deformation above the transformation temperature, an increase in the strength at high temperatures should make them more competitive for applications at high temperature.
Different ways to increase RT (room temperature) hardness and HT (high temperature) strength of silicides have been tested. Thus, molybdenum silicide has been alloyed with V, Ti, Nb, Ta and Al in order to modify the microstructure and thereby increase the strength.
Further, MoSi2xe2x80x94ZrO2-composites have been studied. ZrO2 has three well-defined crystalline conditions, namely monoklinisk (M), tetragonal (T), and cubic (C). The M-phase is stable up to 1170xc2x0 C., where it will be transformed to the T-phase. The T-phase is stable up to 2370xc2x0 C., whereafter the C-phase is stable up to the melting point of 2680xc2x0 C. stabilizers in the form of Y2O3, MgO, CaO or CeO2 have also been added to such materials. By the addition of such stabilizers, the material could be made tougher at a temperature below 1170xc2x0 C.
However, corrosion and oxidation tests have shown that such materials have poorer properties than monolithic molybdenum silicide and material which is commercially available under the trademark KANTHAL SUPER.
The present invention solves the above-mentioned problems, and others, by providing a material having oxidation and corrosion properties which are comparable to monolithic molybdenum silicide or KANTHAL SUPER, but additional has high RT-HV and RT-KC values.
According to a first aspect, the present invention provides a composite material with high strength at both room temperature and high temperatures, the material comprising a mixture including MoSi2; 5-30 vol-% ZrO2; 5-15 vol- % MoB; and an oxygen content, in the form of SiO2 on or in particles of MoSi2, below 0.5 weight-%.